NEW YORK

President Barack Obama earned a rare moment of bipartisan acclaim last month when he commuted the sentences of eight long-serving federal prisoners. Their crack cocaine offenses had resulted in the harsh penalties mandated by a sentencing formula that Congress repudiated when it passed the Fair Sentencing Act of 2010. The old formula, under which possession of a quantity of crack earned the same sentence as possession of 100 times that quantity of powdered cocaine, was "now recognized as unjust," the president said.

But there were ghosts at last month's party: thousands of federal inmates still serving time under sentences that would not have been imposed under the new law. Most are black. As is widely recognized, crack has been the cocaine of choice for African-American users and dealers even as white offenders choose powder. The racially disparate impact of the old law, which dates from the crack-cocaine panic of the mid-1980s with its now-discredited theory that crack was many times more dangerous, made reform a civil rights priority.

These prisoners remain in drug-sentencing limbo. When Congress passed the Fair Sentencing Act, which reduced the crack-to-powder sentencing ratio from 1:100 to 1:18, it was silent on retroactivity. The Supreme Court granted limited relief two years ago, ruling that those who committed their crimes before the law took effect in August 2010, but who were not sentenced until later, could retroactively get the new law's benefit.

In his majority opinion, Justice Stephen G. Breyer said this outcome was consistent with Congress' "language, structure, and basic objectives." But the decision, Dorsey v. United States, said nothing encouraging about broader retroactivity, and the most conservative members of the court wouldn't even have gone that far. The vote was 5 to 4.

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Sens. Richard J. Durbin, D-Ill., and Mike Lee, R-Utah, introduced a bill last summer to authorize judges to grant relief to pre-2010 prisoners on a case-by-case basis. But the Smarter Sentencing Act, as its sponsors call it, has yet to move toward a vote.

There is something that doesn't love retroactivity. It's hard-wired in the American DNA, and embodied in the Fifth Amendment's ex post facto clause, that when things change for the worse -- a harsher sentence, a new crime -- So maybe there's a kind of balance to the notion that on the rare occasions when the criminal justice system becomes more lenient, the effect should be only prospective as well. After all, offenders knew the rules.

And yet there is something too pat about that assumption of equivalency, something grindingly unfair about the plight of those pre-2010 inmates. (As a reminder, the same mandatory five-year minimum sentence resulted from possessing with intent to distribute 5 grams of crack cocaine, less than one-fifth of an ounce, and 500 grams of cocaine powder, more than a pound.)

Society made a judgment, expressed in a bipartisan political consensus, that disparities of this kind were irrational and racially inequitable. Passage of the Fair Sentencing Act was preceded by years of debate, including pleas by federal judges who hated what the law made them do. Gradually, insight emerged. Keeping a known and finite group of people locked in a system acknowledged to be irrational is irrationality itself.

A federal appellate panel last spring went further, finding it not just irrational but unconstitutional. In a 2-1 ruling in United States v. Blewett, the 6th U.S. Circuit Court of Appeals held that because "the discriminatory nature of prior crack sentences is no longer a point of legitimate debate," denying retroactive effect to the new law "would perpetuate proven racial discrimination and thereby violate equal protection."

The Obama administration moved within days to ask the full 6th Circuit to overturn the panel's decision. Citing Supreme Court precedents holding that only intentional discrimination, and not merely a foreseeable discriminatory effect, violates the constitutional guarantee of equal protection, Justice Department lawyers told the 6th Circuit that the panel's decision was based on "a nonexistent equal-protection problem" and was seriously flawed. Last month, the full 6th Circuit overturned the panel decision by a vote of 10-7.

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Every appeals court to reach the issue has now ruled against retroactivity for the Fair Sentencing Act. That means the Supreme Court is unlikely to intervene. But another retroactivity question is looming. There is growing disagreement in the state courts over whether to give retroactive effect to the justices' 2012 ruling in Miller v. Alabama, which invalidated mandatory sentences of life without parole for juveniles who commit murder. The fate of some 2,000 life-sentenced inmates hangs in the balance. So does our own sense of justice.

Linda Greenhouse writes for The New York Times.